During a heavy rainshower, a twinning of the upper part of a rainbow can sometimes be observed, which often lasts from a few seconds up to several minutes. As for a long time there were only few observations of this phenomenon available, only speculations could be made about its origin. Only in the past few years, this twinning could be registered more often by continuous observations, and due to some detailed descriptions, new theories could be advanced. As in all observations both bows are of equal brightness, light refraction on ice particles can be ruled out. Most probably is that raindrops of a non-spherical shape produce one of these bows or even both of them. Due to surface-tension, small rain droplets hardly change their shape when falling, but large drops can be flattened by the air resistance. The more flattened they become, the smaller is their refractive index. So the sunlight has to fall upon water drops of different size at the same time to make the twinning appear. As this twinning was up to now observed under big shower or thunder clouds which formed in hot air,† it can be supposed that the small, not flattened raindrops evaporate at a short distance below the cloud basis. This would explain why the twinning can only be seen for a short time and exclusively in the upper part of the rainbow. It should be important to determine the radius of the rainbow when the twinning is observed, and to record the weather situation at the time the twinning appears as exactly as possible.

Picture: Computer simulation of round and flattened raindrops
at a sun elevation of 24°. Simulation made by Les Cowley.